Digital storage devices such as a HDD-recorder or a DVD-recorder usually have an analog video input for receiving analog audio/video signals from different kind of sources, and an analog video output for displaying the analog video signal on a television set. A typical processing flow of a HDD-recorder or a DVD-recorder is as follows: Analog video input→Video Decoder→MPEG Encoder→MPEG Decoder→Video Encoder→Analog Video Output. After decoding by the Video Decoder the analog video signal is first digitized by means of an Analog-to-Digital converter before any digital processing or compression is performed. The digitized video signal normally conforms to the ITU R-656 standard. The MPEG stream may be stored on a recording medium such as for example a DVD disc or a DVHS tape.
Some problems arise when the video source is a VCR or an un-tuned channel. In case the source is an un-tuned video channel, the video decoder may provide fields with incomplete lines or an incomplete number of lines per field. When the video source is a VCR, and the VCR operates in a trick mode such as fast forward/backward mode, the frame frequency is variable. There are less or more lines per frame than in the normal mode. In addition, in case of an edited tape, the field sequences may be reversed. A normal field order has Upper-Lower-Upper-Lower field ordering. An abnormal field ordering occurs when a consecutive field is the same as the previous field, i.e. Upper-Lower-Lower-Upper-Lower or Upper-Upper-Lower-Upper-Lower. In pause mode, stop mode or double speed mode of the VCR, only Upper fields or Lower fields are provided, i.e. the field ordering is Upper-Upper-Upper . . . or Lower-Lower-Lower . . . .
In all of the above problematic cases the video decoder provides abnormal frame data to the MPEG Encoder chip in the ITU-R BT.656 4:2:2 format. This will make the MPEG Encoder chip choke and lose sync, the pictures become un-smooth, will pause and even freeze.
In this context JP 2002-010217 discloses a video signal processing device capable of solving the above freezing and block noise issue when a non-standard signal is input to a MPEG encoder. A time-axis amendment circuit transforms the input signal into a near-standard signal, and a reset circuit generates a reading synchronization when the field length deviates. A field distinction circuit amends the Upper and Lower order when a non-interlaced signal is input.
U.S. Pat. No. 6,714,717 describes a time base corrector apparatus for removing time base errors from video signals. Time base correction is accomplished by lengthening or shortening each video line in sub-pixel increments by using an interpolator circuit to interpolate consecutive pixels. The interpolator compensates for too few pixels per line by compressing the stretched pixels to the proper size and by creating new pixels from a portion of the pixels immediately before and after the created pixel. The interpolator compensates for too many pixels per line by incorporating the values of the input pixels into the proper number of pixels by an interpolation technique.
U.S. Pat. No. 4,763,203 discloses a digital time base corrector, which is capable of correcting time base errors from video sources operating in trick mode. The corrector has a memory and a dropout detector. When a dropout is detected, the dropout portion of the video signal is replaced by previously stored values.